The present invention relates to water soluble or dispersible epoxy resins which are useful for surface coatings, particularly on metal surfaces such as steel, galvanized metal and aluminum.
Epoxy resins generally have good adhesion and chemical resistance characteristics and are well known for use as protective coatings on metal surfaces. Unfortunately, although it would be advantageous to have water-soluble epoxy resins for use as surface coatings, most epoxy resins are insoluble in water and hence, are applied either as 100 percent solids or by means of an organic solvent. Epoxy resins can be modified to be water-soluble or dispersible but such modification can result in certain disadvantages.
For example, epoxy resins can be emulsified in water using emulsifying agents and applied as coatings. However, use of emulsifiers increases the sensitivity of the coatings to chemical and humidity attack. Alternatively, an epoxy ester can be made from an epoxy resin. The epoxy ester can then be made water dispersible or water soluble by reacting the ester with a base. For example, an epoxy resin can be reacted with a fatty acid or an organic acid, the reaction product of which can then be reacted with an anhydride such as maleic anhydride, phthalic anhydride, succinic anhydride, trimellitic anhydride or the like to generate carboxylic acid functionality and water dispersibility. Unfortunately, the product is sensitive to ester hydrolysis and precipitates upon storage in water.
It is an object of the present invention to provide a process for providing a water soluble or dispersible epoxy resin suitable for use in coating compositions. Another object of the present invention is to provide an epoxy resin which is water soluble or dispersible but has good stability. Still another object of the present invention is to provide an epoxy resin which can be applied as a coating which has good coating characteristics with regard to resistance to attack by chemicals and humidity. These and other objects of the present invention will be apparent from the following disclosure. All parts and percentages herein are percent by weight unless otherwise indicated.